The present invention is directed to a process for making synthetic motor fuel, including hydrogen, motor fuel, gasoline, diesel and fuel oil by processing low-grade coals, heavy still bottoms, phytogenous hydrocarbons, and wood wastes.
Hydrogen is considered to be the most promising fuel of the future (Dunn S., Int. J. of Hydrogen Energy, 2002, V. 27, No. 3, P. 235-264). ‘Hydrogenous society’ of the future shall be based on hydrogen energy, and the major direction hereof is hydrogen generation owing to water decomposition under the effect of sunlight with the further usage of hydrogen in fuel components or as a car fuel.
One of the most serious issues in application of hydrogen as a motor fuel is a selection of a storage method on board of a car vehicle. Hydrogen is the lightest of all chemical elements that is why in the given volume it is stored in much fewer quantities than other kinds of fuel. Thus, at room temperature and standard atmospheric pressure hydrogen occupies approximately 3 thousand times bigger volume than a gasoline of a similar energy quantity. That is why to fill a car with enough quantity of fuel, it is necessary either to discharge hydrogen under high pressure, or to use it in the form of a cryogenic liquid, or to equip cars with sophisticated fuel systems.
The second promising direction of a synthetic motor fuel is a dimethyl ether (DME) generated from a syngas containing hydrogen and a carbon oxide. The major shortcoming of DME usage as a diesel fuel is its aggregative state. Under ordinary conditions it is a gas that is why its storage and transportation may cause problems connected with hydrogen storage and transportation. A wide application of DME may be complicated also by the necessity to carry out profound changes in the infrastructure of filling station chains.
The closer synthetic fuel is a propyl and butyl alcohol mixture (PBAM) generated when deliberately fermenting agricultural commodities such as wheat, barley, corn, etc. The shortcomings of the given process are low productiveness and a high price of the raw material. A wide application of DME and PBMA are restrained by the high price of syngas that is generated mainly from a natural gas (methane). The current processes of syngas generation from carbons are technologically complicated and contaminating environment and require a sufficient capital investment.
The main requirement to a syngas is a high content of hydrogen—not less than 60-80%, vol.—and low content of sulphur—not more than 5-10 ppm. The second component of syngas is a high-toxic substance that leads to high requirements to safety measures of the given process. A high temperature at standard processes of syngas generation is achieved by burning a carbon mass using oxygen or its mixture with air or aqueous vapour as a gasifying agent. The necessity to apply oxygen increases significantly both primary and secondary financial expenditures.
A known a method of syngas generation is SU No 1,686,885, which teaches, when coal-bearing layer processing, including drilling of two well systems to supply an oxidant and discharge of gasification products where a saturated saltpeter solution is pumped down to increase the temperature and efficiency of coal layer processing by means of a diverter well network. The shortcoming of this method is high content of carbon dioxide in a product.
Another well-known method is RU No 2090750, which teaches coal seam uncovering by wells, underground gas-generator ignition and gas supply provision when maintaining supercritical pressure and temperature by means of opening well heads and lowering a water level in dewatering wells. The shortcoming of this method is low content of carbon in gasification products due to supplying air as an oxidant containing big quantities of nitrogen. In the course of gasification NOx nitrogen oxides are developed that disturb the ecological balance.
Yet another well-known method is RU No 22354820 C1, which teaches adding aluminum powder in proportion Al:H2O=1:(4-5) parts of weight under pressure to provide the supercritical water conditions to increase the discharge of a syngas and carbon content into a burned zone of the underground generator. In what connection oxygen is not supplied externally but generated in the burned zone from water according to the reaction. At that hydrogen, carbon monoxide and methane are generated. The shortcoming of this method is complexity and high marginal cost.
US Pat. Publication No. 2009/0206007 discloses the conversion of coal into syngas in a reaction vessel under supercritical conditions (P=300 6ap); the mixture of water and oxygen is used as an oxidant, and sodium hydroxide (0.75%, weight) is used as a catalyst. The process has two stages. At the first stage at the temperature 580-600° C. coal oxidation happens, at the second stage the carbon-dioxide extraction of products is carried out at the temperature of 380-420° C. and under the supercritical pressure. A carbon conversion of 41-60% is achieved, and the process yields gaseous, liquid and solid phases. The gaseous phase contains H2, CO and CO2. The liquid phase contains benzene, toluene, and xylene. Hydrocarbons containing 18-20 atoms of carbon are found in the solid phase. The problems of this method are high product costs, the need of further product purification, and further processing to generate motor fuels.
The gases generated by gasification in accordance with these prior art methods are not suitable for their immediate efficient usage as a fuel for current internal-combustion engine models and require additional processing.
Fischer-Tropsch processes for hydrocarbon synthesis from CO and H2 (syngas) are known to produce gaseous and liquid hydrocarbons as well as oxygenates which, in general, follow the well-known Anderson-Schulz-Flory product distribution.
These reactions can be carried out in fixed, fluidised or slurry bed reactors. The production of olefins and liquid fuels, especially in the gasoline range products, is most favoured by synthesis carried out in a two-phase fluidized bed reactor operating at 350° C. and 20 bar or higher pressures and usually utilizing a fused alkali promoted iron catalyst. This is known as a high temperature Fischer-Tropsch (HTFT) process.
In terms of the ideal Anderson-Schulz-Flory product distribution it is clear that the C5+ selectivity has a maximum value of around 65%. In a commercial high temperature Fischer-Tropsch process performed in a fluidized bed reactor, the optimum C5+ compound yield is usually not realized, thus resulting in a much lower C5+ compound selectivity. The reason for this is that at optimum conditions for the production of maximum C5+ compounds the process is negatively influenced by other factors, one important factor being the formation of elemental carbon. The disadvantage is that the elemental carbon is deposited on the iron catalyst which causes swelling and disintegration of the particles. This powdering of the catalyst particles results in the plugging of fixed bed catalyst reactors. In a fluidized bed the fines which are produced as a result of catalyst disintegration have a high carbon content and hence have a low particle density. Because of this the fines are readily carried out of the reactors by effluent gas and will foul the downstream equipment and also the heavy oil products. Due to the swelling of the individual particles, the entire fluidized catalyst bed expands which negatively influences the reaction.
In order to reduce carbon formation one can reduce the levels of alkali promoter for the iron catalyst and one can also increase the H2:CO ratio in the syngas to be converted to hydrocarbons. However, it is known that iron based Fischer-Tropsch catalysts with a low alkaline promoter level tend to produce light hydrocarbons and are also not easily converted to the C5+ compound range. That is, it has been found that a HTFT process with less than 0.02 mol alkali metal promoter per 100 g iron (even if a H2:carbon oxide ratio of less than 2 is used) favors products in the C1 to C4 compound range as have been published in Catalysis Science and Technology, Volume 1, 1981, pages 202-209 and WO 0197968.
The situation is accordingly that if conditions are selected (either a low alkali level or a high H2:CO ratio) to reduce formation of elemental carbon, it is expected that hydrocarbon products in the C1 to C4 range will be favored, thus a synthesis hydrocarbon product with less than 30% by weight of C5+ product will form.